Something that's a cinch is incredibly easy. It's a cinch to eat a meticulously decorated cake — it's much more complicated to bake and frost one.
Cinch is one of those words with many meanings that seem unrelated at first glance. The original 19th century North American definition, which is still used today, is "saddle girth," the straps that keep a horse's saddle in place. As a verb, cinch means "to pull tight," the way you'd cinch a belt. Informally, to cinch is to make absolutely certain: "It'll cinch her college decision if that school offers a scholarship."